Such a photo acoustic detector is known from the United States patent application, published as US 2005/0117155. Said patent application describes a photo acoustic trace gas detector using a quartz tuning fork for detecting the pressure variations. Light modulation is performed using amplitude or wavelength modulation. After amplification by a pre-amplifier, a lock-in amplifier mixes the detector signal with a reference signal for acquiring an output signal. The reference signal for the lock-in amplifier is taken from a signal for modulating the light beam. The use of the quartz tuning fork for the detection of the pressure variations allows for a relatively compact photo acoustic trace gas detector.
An application of photo acoustic trace gas detectors is breath testing. Breath testing is a promising area of medical technology. Breath tests are non-invasive, user friendly and low cost. Prime examples of breath testing are monitoring of asthma, alcohol breath testing and detection of stomach disorders and acute organ rejection. First clinical trials show possible applications in the pre-screening of breast and lung cancer. These volatile biomarkers have typical concentrations in the parts per billion (ppb) range. Nitric oxide (NO) is a well known trace gas in the human breath, and elevated concentrations of NO can be found in asthmatic patients. Currently, exhaled NO levels at ppb concentrations can only be measured using expensive and bulky equipment based on chemiluminescence or optical absorption spectroscopy. A compact, low-cost NO sensor forms an interesting device that can be used to diagnose and monitor airway inflammation and can be used at the doctor's office and for medication control at home.
It is a problem of the photo acoustic trace gas detector according to US 2005/0117155 that the detector currents during trace gas detection are often very low and easily dominated by electronic noise, which limits the trace gas detection at low concentrations.